fix: Add getServiceName() to EnhancedBigTableStubSettings

[lqiu96]

Adds a duplicate getServiceName() implementation to EnhancedBigTableStubSettings. This is similar to

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/BigtableStubSettings.java

Lines 171 to 174 in de639bc

	@Override
	public String getServiceName() {
	return "bigtable";
	}

, but is re-implemented as EnhancedBigTableStubSettings does not implement BigtableStubSettings